The invention relates to a catalytic method for the production of 2-hydroxy-4-(methylthio)butyronitrile (MMP-CN) from 3-(methylthio)propanal (=methylmercaptopropionaldehyde, MMP) and hydrogen cyanide (HCN). In particular, the invention describes a process for synthesizing storage-stable MMP-CN using stoichiometric amounts of prussic acid (HCN), the product containing superstoichiometric amounts of prussic acid, in relation to the unreacted MMP or to the MMP in equilibrium with MMP-CN.
2-Hydroxy-4-(methylthio)butyronitrile (MMP-cyanohydrin) is an intermediate for the synthesis of D,L-methionine and the methionine hydroxyl analog 2-hydroxy-4-methylthiobutyric acid (MHA). Methionine is an essential amino acid which is used, inter alia, as supplement in feedstuffs. MHA is a liquid methionine substitute having low bioavailability.
From MMP, by reaction with hydrogen cyanide (prussic acid), MMP-cyanohydrin (2-hydroxy-4-(methylthio)butyronitrile) may be produced using suitable catalysts. Suitable catalysts are, e.g., pyridine or triethylamine. By hydrolysis of MMP-cyanohydrin with, e.g., mineral acids, MHA is obtained. Methionine is formed by reaction of MMP-cyanohydrin with ammonium hydrogencarbonate, with formation of hydantoin, which can be saponified with a base, e.g. potassium carbonate or sodium hydroxide. Methionine is liberated with carbon dioxide or sulfuric acid.
It is known, for example, from the U.S. Pat. No. 4,960,932, to produce methionine by a four-stage method. In the first step, by addition of HCN to MMP in the presence of triethylamine, the MMP-cyanohydrin is produced. The amount of HCN used corresponds to 1.05 mol in relation to the amount of MMP used. Then, the MMP-cyanohydrin, in a second step, is reacted with ammonia, whereby 2-amino-4-methylthiobutyronitrile is formed which, in a third step, is then hydrolyzed in the presence of a ketone and an alkali metal hydroxide, forming methylthiobutyramide which is finally saponified to form an alkali metal methioninate.
In the case of production of 2-hydroxy-4-methylthiobutyric acid (MHA), the 2-hydroxy-4-methylthiobutyronitrile is obtained by reacting MMP and HCN in a medium that contains pyridine or an amine (see U.S. Pat. No. 2,745,745, column 2, lines 52 to 55). Excess HCN is merely distilled off, e.g. in a vacuum. The resultant 2-hydroxy-4-methylthiobutyronitrile is then hydrolyzed with sulfuric acid, whereby the amide of 2-hydroxy-4-methylthiobutyric acid is directly formed, and finally 2-hydroxy-4-methylthiobutyric acid is formed. A similar method is also described in EP A 330 527 A1 or in U.S. Pat. No. 4,912,257.
In addition, in WO 96/40631 A1, the production of MMP-cyanohydrin by reacting MMP with hydrogen cyanide in the presence of a suitable addition reaction catalyst is described. There it was found that triisopropanolamine, nicotinamide, imidazole, benzimidazole, 2-fluoropyridine, poly-4-vinylpyridine, 4-dimethylaminopyridine, picoline and pyrazine can serve as addition reaction catalysts for producing MMP-cyanohydrin. Furthermore, trialkylamines having three to eighteen carbon atoms in each of the alkyl substituents bound to the nitrogen atom and tertiary amines in which at least one of the non-hydrogen substituents that are bound to the nitrogen atom according to the above description contains an aryl group can also serve for catalyzing the reaction between MMP and hydrogen cyanide to form MMP-cyanohydrin.
Preferably, in this case, the hydrogen cyanide is used in a molar excess of about 2%, based on MMP.
WO 2006/015684 A2 finally discloses a method for, in particular, continuous production of MMP or of MMP-cyanohydrin in which in each case heterogeneous amine catalysts are used for the addition reaction.
In addition, it is known from the U.S. Pat. No. 5,756,803 to react an aldehyde with hydrogen cyanide in the presence of a buffer, by means of which the pH of the solution can be set above 4, amines being excluded. Quite generally, as buffer, mixtures of alkali metal salts of acids and acids, or mixtures of acids and alkali metal hydroxides can be used. The buffer is used in order firstly to avoid the decomposition of the starting materials and of the desired product and secondly to neutralize the acids used for stabilizing hydrogen cyanide. Likewise, here, HCN is added in a molar excess to the MMP, the molar excess preferably being in the range from 2% to 5%. In the reaction of MMP with HCN in the presence of the customarily used bases, although these increase the reaction rate under the conditions specified, they rapidly lead to a decomposition of the cyanohydrin formed and to decomposition of the aldehyde used at the start, forming a highly discolored solution.
In order to recover the residual amounts of unreacted HCN and MMP contained in the exhaust gas of the reactive absorber, said unreacted HCN and MMP are scrubbed using a water scrubber, the scrubbing water passing into the product. The water content in the product is approximately 48% by weight.
The substantial disadvantages of the methods previously described in the literature are that for achieving a high MMP-CN yield, high molar excesses of HCN have to be used. The excess amounts of HCN are lost in the methods described and are a great economic disadvantage. Furthermore, the catalysts used in the methods described also promote the formation of unwanted byproducts from the aldehydes used, which lead to contamination of the product which cannot be tolerated. One approach to solving this problem of formation of byproducts is described in U.S. Pat. No. 5,756,803 with, of course, large amounts of water passing into the product, which firstly, for production of methionine, need to be at least partially removed and which secondly again promote the decomposition of the MMP-cyanohydrin which in each case is a not inconsiderable disadvantage. Therefore, the product described in U.S. Pat. No. 5,756,803 is not storage stable and, for storage and in particular for transport, must be processed in a complex manner by means of removal of the water by distillation, which is a great economic disadvantage of the method.